1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device using a crystalline semiconductor mainly comprising silicon. The silicon semiconductor obtained according to the method of the present invention is suitable for use in thin film transistors, thin film diodes and the like.
2. Prior Art
Thin film transistors utilizing a semiconductor thin film (referred to simply hereinafter as “TFTs”) are well known. TFTs are fabricated by forming a thin film semiconductor on a substrate and processing the thin film semiconductor thereafter. TFTs are widely used in various types of integrated circuits, and are particularly suitable in the field of electro-optical devices such as liquid crystal displays; more specifically, as switching elements provided for each of the pixels in an active matrix liquid crystal display device as well as the driver elements of peripheral circuits thereof.
An amorphous silicon film can be utilized most readily as a thin film semiconductor for the TFTs. However, there is a problem in that the electrical characteristics of the amorphous silicon film are poor. The use of a thin film of crystalline silicon can solve this problem. The crystalline silicon film can be prepared by first forming an amorphous silicon film and then heat treating the film to crystallize it. Otherwise, high energy electromagnetic waves for example, a laser beam, can be radiated onto the amorphous silicon film.
The heat treatment for crystallizing amorphous silicon film requires heating the film at a temperature of 600° C. or higher for a duration of 10hours or longer. Such a high temperature heat treatment is undesirable for a glass substrate. For example, Corning 7059 glass, commonly used for substrates of active matrix liquid crystal display devices, has a glass distortion point of 593° C. and is therefore not suitable for large area substrates that are subjected to heating at a temperature of 600° C. or higher.
According to studies carried out by the inventors of the present invention, it has been found that crystallization of an amorphous silicon film can be effected by heating the film at 600° C. or less, e.g. 550° C. for a duration of about 4 hours. This can be accomplished by first disposing a trace amount of nickel or palladium, or other elements such as lead in contact with the amorphous silicon film and then applying a heat treatment at the above temperature for crystallization.
It has also been known to utilize a laser for crystallization. The silicon film obtained by this method is suitable for use in the fabrication of a TFT since it yields superior characteristics such as high field mobility, low S value, and low threshold voltage. However, the crystallinity of the silicon film thus obtained strongly depends on the energy of the laser. It is therefore apparent that, due to the instability of laser energy, it is very difficult to stably obtain a crystalline silicon film with a high reliability.
Further, in the case of crystallizing an amorphous silicon film with a silicon oxide in contact therewith, the resulting silicon film tends to be oriented along the (111) plane. A TFT having a channel forming region having such a crystal orientation tends to have a threshold voltage Vth shifted to a negative value due to the large positive fixed charges. Such TFTs are unfavorable in constituting a complementary circuit composed of an N-channel TFT and a P-channel TFT. Accordingly, in order to control the shift in the threshold voltage, the silicon film must be fabricated with care that it does not orient along the crystallographic (111) plane.